Compliant diaphragm for planar magnetic transducers

ABSTRACT

Planar magnetic transducers and acoustic speakers incorporating the transducers wherein improved frequency performance with lower distortion of acoustic diaphragms of the transducers is obtained by forming the diaphragms of film materials ( 20 ) which are significantly more compliant than materials used to form electrical circuit patterns ( 30 ) on the diaphragm film materials ( 20 ).

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional PatentApplication Ser. No. 60/402,916, filed Aug. 14, 2002, in the name of thesame inventor.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention is directed to the field of planar magneticacoustic transducers and, more particularly, to the use of compliantfilms having metallic electrical trace patterns or circuits adhesivelysecured thereto to form sound producing diaphragms having improved lowfrequency responses and lower distortion levels and which are suitablefor use in small panel type transducers.

2. Brief Description of the Related Art

Audio system markets desire small flat transducers with improved lowfrequency output, reduced distortion and enhanced efficiency. In a“Wireless World” article by Peter Walker, limitations are disclosed withrespect to electrostatic loudspeakers, which limitations also apply toother types of tensioned film loudspeakers. The diaphragm mass limitshigh frequency performance, air load limits mid range performance anddiaphragm stiffness limits low frequency performance. In anelectrostatic loudspeaker, stiffness of the diaphragm is desirablebecause of the unstable situation that exists. When the polarizingvoltage is high the diaphragm will be attracted to one stator. If thediaphragm compliance is high enough and a high polarizing voltage isused, a speaker diaphragm will fall in or collapse against one stator.

The foregoing is not a consideration in a planar magnetic or ribbontransducer because in an idle state of the transducer, there are nopotential forces acting on the transducer diaphragm. However, thestiffness of the diaphragm is a greater factor because the diaphragmincludes at least a conductor material. Favorable conductor materialsinclude copper, aluminum and silver. In a planar magnetic speaker, thematerial of choice is a soft alloy aluminum. The conductor material isadhered or laminated to a film substrate, usually polyester. The sum ofa compliance value of the conductor material and a compliance value ofthe film material define the overall diaphragm compliance. Also thelayout of the conductor trace pattern and the ratio of conductorcoverage to substrate film material affects the overall stiffness. Thedefinition of compliance as described herein, is applied to thediaphragm material compliance and elasticity and not to the acousticcompliance. Compliance is thus a measure of elasticity or amount ofelongation of a material where a strain of an elastic body is expressedas a function of a force producing the strain.

Historically, high compliance diaphragms have not been used in planarmagnetic transducers due to several factors including limited availableprocesses for laminating and etching circuits on suitable highcompliance films, and limited focus on reducing transducer size bymodifying the diaphragm. Most planar magnetic products were designed forhome audio speakers and reduced area transducers were not a traditionaldesign criteria. While there have been dual material diaphragms for fiatspeakers incorporating a compliant surround around a driven area whichresponds like a rigid piston, typically an unsuitably high Q results atresonance and low frequency response is not improved significantly.

Resonance and modal patterns occur on the surface of a diaphragm thatare usually much smaller than the wavelength of the sound being producedwhereas, with high stiffness materials, the modal pattern amplitudes areincreased. At some frequencies, modal patterns result in non linearmotion of the diaphragm which causes distortion.

Low frequency limitations of a stretched membrane or diaphragmloudspeaker are further limited by tension. When the tension is reducedbelow a certain level, usually about one pound per square inch, loss ofpiston like motion occurs and non harmonic forms of distortion occur.This prevents small diaphragms from achieving low frequency capabilitybecause the necessarily high tension results in a high fundamentalresonance. For example, with tension on the order of one pound persquare inch, a rectangular diaphragm of approximately 5 by 7 inches willyield a resonance frequency of about 120 Hz and a diaphragm ofapproximately 7 by 20 inches yields a resonance frequency of about 60Hz. The lower tension limit restricts the ability of the transducer tohave significant output below these frequencies because of the highstiffness of the diaphragm.

An alternative approach of a multiple layer diaphragm increases thestiffness and mass thereby decreasing efficiency and low frequencyoutput. Additional conductor layers increase energy potential but addmass and stiffness generally canceling the benefit of additionalconductor length in a gap for a wide range transducer.

SUMMARY OF THE INVENTION

It is an object of the invention to improve the low frequency responseand distortion levels of a planar magnetic transducer by employingcompliant films with the proposed techniques in the application. Theresonance frequencies in the above speaker diaphragm examples can bereduced by at least a factor of two or more, using the diaphragm of theinvention. With high compliance films, resonance frequencies of 30 Hzcan be achieved on the small panel example (5″×7″) and 20 Hz or lower onthe larger panel example (7″×20″). The reduced resonance representssignificant improvement over known designs. Hence a transducer using theinvented diaphragm, can operate at a much lower resonant frequency thana standard planar magnetic transducer of similar size and traditionaldiaphragm design. To state this another way, it is an object of theinvention to reduce transducer size. A planar transducer using thediaphragm of the present invention can be much smaller in area than atraditional planar magnetic transducer diaphragm when the resonantfrequencies of both are similar. This enables new applications andsystem designs using this type of planar magnetic transducer diaphragm.

In the present invention, a compliant material is used as a base filmlayer for a transducer diaphragm and to which is adhesively secured orlaminated a metallic electrical circuit trace pattern. A ratio of acompliance value or ultimate elongation of the film material over acompliance value or ultimate elongation of the metallic conductormaterial must be substantially greater than one and preferable greaterthan approximately forty (40).

It is an object of this invention to reduce the size of planarelectromagnetic transducers by improving low frequency performance andlowering distortion.

BRIEF DESCRIPTION OF DRAWINGS

A better understanding of the invention will be had with reference tothe accompanying Drawings wherein:

FIG. 1 shows a planar magnetic transducer with high compliancediaphragm;

FIG. 2 is a comparison of distortion performance for a) high compliancesubstrate of Urethane and b) standard Mylar® substrate showing reduceddistortion; and

FIG. 3 single sided planar magnetic transducer with high compliancediaphragm.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Diaphragms designed for use with planar magnetic speakers typically areone thousands of an inch in thickness and include a laminated electricalcircuit trace pattern which is designed to provide the electromechanicaloperation, as is commonly used in the art. The conductor trace layoutand spacing is designed for locating in areas of maximum magnetic fieldstrength provided by driving magnets as is well known in the art.

A preferred embodiment of high compliance diaphragm 10 for a planarmagnetic transducer or speaker 70 is shown in FIG. 1 as being tensionedand located between opposing support frames 60 and 62 of the transducerstator. An open area of the diaphragm which is movable between theframes is known as the active surface area of the diaphragm. Thediaphragm is mounted between opposing magnetic motors 35 and 36 each ofwhich includes magnetic elements 50. As shown, the magnetic elements arepreferably aligned with one another on opposite sides of the diaphragmwith like poles opposing one another. The spacing or open area in theframes between of magnetic elements is provided for passage of soundwaves created by vibration of the diaphragm when electrical energy isapplied to a metallic conductor circuit pattern 30 which is applied to asurface of a high compliance substrate film 20. Examples of suitablefilm materials include Urethane, Tefzel® and Teflon® polymer thin films,however, non-polymer compliant thin films can also be used such asNylon® or Lycra®, provided a suitable air seal is maintained and theconductor material suitably attached.

Ultimate elongation of typically used materials such as Mylar®, Kapton®or Kaledex® reduces the ability to have large diaphragm excursionsbecause of limited ultimate elongation and compliance. Below resonance,the force required to stretch the material is greater than the force tomove air within the transducer or the force available from the motorcircuit reducing output. Additionally, as it is desirable for lowfrequency performance to have compliance in both axes in a plane of adiaphragm 10, the way to achieve this is through use of the substratefilm 20 which has a significantly higher compliance or ultimateelongation than a metallic conductor material 30 applied to the filmsubstrate.

The conductor traces 30 are shown attached to the high compliancediaphragm film 20 by a very thin adhesive layer 40. In a planar magneticspeaker, the material of choice for the conductor trace 30 is a softalloy aluminum. Other conductor materials mentioned in the introductioncan be similarly used in the invention. For many audio products,transducer dimensions are typically rectangular with aspect ratios onthe order of 2:1 and higher. Because of the mechanical characteristic ofa stretched film diaphragm, the width or narrow dimension of thetransducer defines the resonance frequency.

Conductor runs are typically lengthwise on a transducer to minimizeresistive losses from turns at the ends of the runs. As the transducerarea is made smaller, the compliance of the conductor runs 30 becomes adominant factor that is adjusted for by the high compliance substrate20. The adhesive layer 40 is typically much thinner than the substratefilm 10 or the conductor traces 30, hence its mechanical properties donot significantly effect the results and operation of the diaphragms ofthe invention.

Additionally, the diaphragm design can be applied independent of magnetmotor structure, and can operate with conventional magnet configurationssuch as NSNS orientation or pole-piece motor structures. The diaphragmsof the invention can also be applied independent of magnet material, andpreferably are used with rare-earth permanent magnets such as Neodymium.

The high compliance material substrate 20 between the conductor traceruns increases the output of a flat panel stretched membrane loudspeakerby allowing a smaller percentage of the total force available to workagainst stretching the diaphragm 10 and a larger percentage of the forceto move the surrounding air. The force is that created between themagnets on one or both sides of the diaphragm and the electrical fieldcreated when energy is applied to the conductor trace pattern. By usingthe techniques disclosed herein, significant increases in transduceroutput have been demonstrated. In combination, the conductor pattern,and the substrate compliance can allow the conductor to undergo largeexcursions and stay positioned within the best field portion of themotor magnetic circuit.

With conventional substrates, such as polyester, which has an elasticmodulus of 500×10³ psi and ultimate elongation of 100% (not shown) as anelectrical signal is applied to the conductor trace pattern, forces onthe conductors result in a curved or parabolic displacement of thediaphragm material between clamped edges of the rigid diaphragm frames.Acoustic output of the speaker is maximum when the center of the curveddiaphragm hits the stator frames on either side of the diaphragm. As thestiffness of the diaphragm material is reduced, the conductors are nolonger constrained by the motion of the substrate. At very lowfrequencies, by adjusting the the compliance across the surface, motionof the diaphragm approaches that of a piston resulting in greateracoustic output for a given sized transducer.

The diaphragms of the present invention exhibit lower distortion andsmoother frequency response than conventional planar magnet transducers.As a diaphragm undergoes displacement, bending of the diaphragm materialback and forth can set up longitudinal waves that travel along thesurface of the diaphragm 10 and reflect off of the diaphragm frameboundary.

The diaphragm substrate material 20, its compliance and the complianceof the conductor traces 30 have a significant influence on distortionthat results from waveforms which set up on the surface of the diaphragm10. Both harmonic and time distortions can be measured as a result ofthis property of a flat loudspeaker diaphragm. In a planar magnetictransducer, transverse waves on the diaphragm are undesirable, oftenresulting in notches in frequency response due to phase cancellation.The high compliance substrate 20 results in a reduction in amplitude ofsurface wave nodes (not shown) and hence lower distortion, due to boththe mechanical decoupling of the conductor traces 30 from the substrate20 and internal damping by the substrate 20. An additional function ofthe invention is that the compliant substrate reduces the velocity ofthe surface waves so that phase cancellation from the reflected waves isshifted to lower frequencies below the useful operating frequencyresponse, hence smoothing the frequency response of the diaphragm in theoperating range of the transducer.

FIG. 2 shows a plot of distortion curve for two diaphragms mounted incomparable transducers but with different diaphragm substrates, Mylar®“M” versus urethane “U” (high compliance). A dramatic reduction indistortion is noticed near 1 kHz using the high compliance diaphragmsubstrate “U”. Test results show that where the diaphragm substratecompliance is greater than the compliance of the metallic material orfoil 30 adhered to the substrate material, and preferably when theultimate elongation of the film substrate is greater than approximately40 times that of the metallic material, distortion is greatly reduced.That is, the ratio of the ultimate elongation “A” of the diaphragmsubstrate film material over the ultimate elongation “B” of the metallictrace material, should be generally greater than 40 (A÷B≧40). Aspreviously noted, the compliance factor or value is directly related toa material's elastic modulus, mechanical stretchability and ultimateelongation.

Another embodiment of the invention is shown in FIG. 3. In thisembodiment, the transducer shown is a single-sided planar magnetictransducer or speaker 80 having driving magnets 50′ provided along onlyone side of the diaphragms. The high compliance diaphragm 10′ is placedin the planar magnetic transducer and is tensioned so as to be locatedabove the one magnet motor structure 35′ that includes the magnets 50′mounted on support frame 60′ opposite support frame 62′. A substratefilm 20′ having a high compliance is shown and is terminated at theframe edges as is standard in the industry. Examples of suitablematerials include Urethane and Teflon® polymer thin films, however, toone skilled in the art, non-polymer compliant thin films can also beused such as Nylon® or Lycra® provided a suitable air seal ismaintained. A metallic electrical conductor trace 30′ is adhesivelysecured to the substrate film in a conventional manner. Ultimateelongation of typically used materials such as Mylar®, Kapton®, orKaledex® reduces the ability to have large diaphragm excursions becauseof limited ultimate elongation and compliance. Below resonance, theforce required to stretch the material is greater than the force to movethe air or available from the magnetic motor circuit thereby reducingoutput. Additionally, it is desirable for low frequency performance tohave compliance in both axes in the plane of the diaphragm 10′. This isachieved through the substrate 20′ having a significantly highercompliance than the conductorterial 30′.

The foregoing description of the preferred embodiments of the inventionhas been presented to illustrate the principles of the invention and notto limit the invention to the particular embodiments illustrated. It isintended that the scope of the invention be defined by all embodimentsencompassed within the following claims and their equivalents.

1. In planar magnetic transducer including a composite diaphragm formed of a thin film having a metallic material electrical circuit trace pattern on a surface thereof and wherein the diaphragm is mounted within a stator frame such that an active area of the diaphragm having the circuit trace pattern thereon is opposed by at least one magnetic motor driver, the improvement comprising, the film having a high ultimate elongation such that a ratio of ultimate elongation “A” of the film divided by ultimate elongation of the metallic material of the electrical circuit trace pattern is generally greater than approximately
 40. 2. The planar magnetic transducer of claim 1 wherein at least one magnetic motor driver is provided on each of opposing sides of the diaphragm within the stator frame.
 3. The planar magnetic transducer of claim 1 wherein the film is selected from a group of materials consisting of urethane, Tefzel®, Teflon®, Nylon® and Lycra® materials.
 4. The planar magnetic transducer of claim 1 wherein a configuration of the electrical circuit trace pattern and the ultimate elongation “A” of the film causes the composite diaphragm to move in a piston-like manner within the stator frame when electrical power is supplied to the electrical circuit trace pattern to thereby increase acoustic output.
 5. The planar magnet transducer of claim 1 wherein the position of the electrical circuit trace pattern and the ultimate elongation “A” of the film is such as to maintain the electrical circuit trace pattern in alignment with a magnetic field created by the at least on magnetic motor driver to thereby increase acoustic output.
 6. The planar magnetic transducer of claim 1 wherein an ultimate elongation of the composite diaphragm within the frame is non-uniform across its surface to thereby cause the composite diaphragm to move in a piston-like manner within the stator frame when electrical power is supplied to the electrical circuit trace pattern and thus increases acoustic output.
 7. An acoustic speaker incorporating the planar magnetic transducer of claim
 1. 8. A method of improving low frequency performance of a planar magnetic transducer including providing a frame defining a central open area and mounting within the frame a composite diaphragm formed of a thin film having a metallic material electrical circuit trace pattern applied thereon and wherein the film has an ultimate elongation which is substantially greater than an ultimate elongation of the metallic material and wherein the circuit pattern is applied to the film in such a manner that the composite diaphragm moves with a piston-like motion within the frame when electrical power is supplied to the circuit pattern.
 9. The method of claim 8 wherein the ultimate elongation of the film is selected to be at least approximately forty times that of the ultimate elongation of the metallic material.
 10. A planar magnetic transducer having improved low frequency performance, the transducer including a frame in which a composite diaphragm is mounted so as to establish an active area within the frame and wherein the composite diaphragm includes a thin film having a metallic material electrical circuit trace pattern on a surface thereof, at least one magnetic motor driver mounted with the frame for creating a magnetic field, and wherein an ultimate elongation of the active area of the composite diaphragm within the frame is non-uniform across its surface to thereby cause the active area of the composite diaphragm to move in a piston-like manner within the frame when electrical power is supplied to the electrical circuit trace pattern. 